Can you add more loop subdivides later?
Yes and no.

One thing you can do is this:


Switch to edge select mode (&#9096; Ctrl&#8633; Tab)
Select an edge ring (&#9096; Ctrl&#9095; Alt RMB)
Multi select edge loops (Fastest to access via the Space menu)
Subdivide


Example:



However this only lets you double the number of existing loops.

Without deleting loops/other manual tricks, I don't think there is currently a way of getting a precise number of evenly distributed loops, especially on more complex objects.

See Is there any way to edit parameters for an object after it is created?